Indulgences do not pardon sins.
Your statement is only partially correct.
“In the Sacrament of Baptism not only is the guilt of sin remitted, but also all the penalties attached to sin. In the Sacrament of Penance the guilt of sin is removed, and with it the eternal punishment due to mortal sin; but there still remains the temporal punishment required by Divine justice, and this requirement must be fulfilled either in the present life or in the world to come, i.e., in Purgatory. An indulgence offers the penitent sinner the means of discharging this debt during his life on earth.”
http://www.newadvent.org/cathen/07783a.htm
Some of us find it odd to say, “I forgive you, but you must now do something for me...” That isn’t grace, but business!